Origin of Life
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Direct answer
Life on earth is explained by chemical evolution preceding biological evolution. Oparin of the Soviet Union and Haldane of England proposed that the early earth — with a hot, reducing atmosphere containing methane, ammonia, hydrogen and water vapour, energised by lightning and ultraviolet radiation — allowed simple molecules to combine gradually into complex organic molecules and finally into self-replicating ones. Stanley Miller confirmed the plausibility of this in 1953 by passing electric discharge through a closed flask of the same gases at 800 degrees Celsius and recovering amino acids, while Louis Pasteur had earlier shown that life arises only from pre-existing life.
What you must remember
- The universe: the Big Bang, about 20 billion years ago, created matter and an expanding universe; the earth formed much later, roughly 4.5 billion years ago, with conditions utterly unsuited to present-day life.
- Against spontaneous generation: Louis Pasteur's careful experiments demonstrated that life comes only from pre-existing life, ruling out the sudden appearance of organisms.
- Oparin-Haldane condition: a reducing atmosphere with methane, ammonia, water vapour and hydrogen, high temperature and violent electrical storms; no free oxygen existed.
- Energy sources: ultraviolet radiation, lightning and cosmic rays powered the first synthesis of organic molecules.
- Miller's experiment (1953): electric discharge in a closed flask containing methane, hydrogen, ammonia and water vapour at 800 degrees Celsius; amino acids formed within a week, showing abiogenesis of organic monomers is possible.
- Corroboration: later experiments with similar setups produced nitrogen bases, sugars and lipids, and meteorite analyses have revealed the same classes of compounds, suggesting such chemistry occurs elsewhere too.
- First life: the earliest, non-cellular forms — aggregates of RNA, proteins and polysaccharides — appeared perhaps 3 billion years ago; cellular forms arose later, in water, followed slowly by photosynthesis and the oxygenation of the atmosphere.
Common confusion
Chemical evolution and biological evolution are sequential, not competing, explanations — chemistry first built the molecules of life, and natural selection later operated on self-replicating systems. Miller created amino acids, not living cells or bacteria; his experiment models prebiotic synthesis only. Remember also that the primitive atmosphere was reducing (oxygen-free), the opposite of today's oxidising air.
Exam-focused takeaway
NEET-UG asks who proposed chemical evolution (Oparin and Haldane), the exact gases and temperature in Miller's flask, the energy source he simulated, and the product obtained (amino acids). Statement items test Pasteur's rejection of spontaneous generation, the reducing nature of the primitive atmosphere, and the sequence non-cellular forms before cellular forms in water. Keep the two billion-year figures for the earth's age and the first life separate and exact.
Frequently asked questions
Who proposed the theory of chemical evolution?
Oparin of the Soviet Union and Haldane of England, who argued that the earth's reducing atmosphere allowed simple molecules to evolve into complex organic ones.
What did Miller create in his 1953 experiment?
Amino acids, by subjecting a mixture of methane, ammonia, hydrogen and water vapour at 800 degrees Celsius to electric discharge in a sealed flask.
What was the composition of the primitive atmosphere?
Methane, ammonia, water vapour and hydrogen — a reducing mixture with no free oxygen.
How did Pasteur's work bear on the origin of life?
He demonstrated that life arises only from pre-existing life, disproving the idea that organisms appear spontaneously from non-living matter.
What were the first non-cellular forms of life like?
Aggregates of RNA, protein and polysaccharides that arose perhaps 3 billion years ago, predating the first cellular forms that appeared in water.